1. Field of the Invention
The present invention relates to a switched capacitor circuit which is preferably used in a filter circuit and the like.
2. Description of the Related Art
FIG. 8 shows an example of a conventional switched capacitor type integrator which includes a switched capacitor circuit. This integrator is composed of a capacitor 50, four analog switches 51, 52, 53 and 54, an operational amplifier 55 and a capacitor 56 for integration.
The analog switches 51, 52, 53 and 54 are respectively C-MOS (Complementary-MOS) transistors shown in FIG. 9. The analog switches 52 and 53 are turned on or off in response to a clock signal .PHI.1 shown in FIG. 10, and the analog switches 51 and 54 are turned on or off in response to a clock signal .PHI.2.
Electric charges are charged into the capacitor 50 when the analog switches 51 and 54 are turned on in response to the clock signal .PHI.2. The electric charges are then transferred into the capacitor 56 when the analog switches 52 and 53 are turned on in response to the clock signal .PHI.1. At that time sampling noise shown in FIG. 10 is generated at an output terminal Vout.
In order to reduce such a sampling noise, an improved SCF (Switched Capacitor Filter) shown in FIG. 11 is proposed in Japanese patent application laid-open No. 58-198918. The definition of the SCF includes the above-mentioned switched capacitor type integrator. The output of the SCF 57 is connected with a low-pass filter 58, such as an RC active filter, to reduce the sampling noise.
However, according to the SCF shown in FIG. 11, additional electronic elements, namely a resistor, a capacitor and an operational amplifier or the like are necessary to compose the low-pass filter 58. Since the resistor and the capacitor occupy the large area, they cannot be generally built in a monolithic integrated circuit in which the SCF is to be formed, and, therefore, are forced to be externally formed in the area other than the integrated circuit. When they are externally formed, manufacturing cost of a whole circuit comes to be raised.